Earth is about 150 million km away from the Sun, a distance defined as 1 AU. Because our orbit is formed within the Goldilocks Zone, water exists in a liquid state, and we have a suitably mild climate perfect for life—all thanks to this sweet spot of a distance. But finding a planet with these perfect, suitable conditions isn't easy. In many sci-fi stories, simply terraforming a planet makes it magically transform into Earth, but if you think about it, that might actually be harder than creating an artificial planet. That's why various structures designed to artificially recreate the Earth environment in space have been dreamt up, and what I’m introducing today is one of them.
This is the [Ringworld]. It's a massive space habitat created by using the distance from Earth to the Sun as a radius, forming Earth's orbit into a giant ring, and then building land and sea on top of it, decorated just like a planetary surface. Its surface area reaches 3 million times that of Earth, and since it's the same distance away as Earth, you don't have to worry about too much or too little sunlight.
The Ringworld concept first appeared in a novel published in 1970 by American author Larry Niven. As a bonus, this author dude was also the first person to use mana as the driving force when casting magic in fantasy.
This gigantic ring spins at a speed of 1,200 km/s (40 times the speed of Earth's orbit) to generate artificial gravity using centrifugal force. Instead of rotating like a planet, it creates a day/night cycle by rotating sunshades called Shadow Squares on the inner surface to block sunlight. This means the Ringworld doesn't have a concept of sunset. The sun is always directly overhead on the Ringworld, and night falls as the sunlight suddenly disappears, like a massive eclipse is happening. Gigantic barriers are installed on both edges of the ring to contain the atmosphere. They are a staggering 1,600 km high, ensuring the atmosphere stays inside the ring and isn't lost into the wide-open sky due to centrifugal force.
The width of the Ringworld is about 1.6 million km. However, because the Ringworld is such a massive structure, the ring structure visible in the sky from the surface looks like an extremely thin, extended line. Its size is roughly equivalent to a full moon stretched out into a band. Even if it's not thread-thin, you can roughly guess how thin it would look. Also, due to this scale, a horizon would form normally on the Ringworld. The ring structure visible beyond the horizon would be blurred by Rayleigh scattering in the atmosphere, meaning the ring wouldn't be visible near the horizon. But here's the weird part: if you raise your gaze only about 5 degrees from the horizon, the layer of atmosphere the light has to pass through thins out, and the appearance of the ring, which had definitely disappeared, suddenly reappears, passing overhead. In other words, if you trace the ring downwards from the sky to the ground, it suddenly vanishes close to the land. The photo above also shows this effect—the ring gets clearer the higher up you look and hazier as you look towards the ground. The closest parts aren't visible, but points hundreds of millions of km away are clear. This is because the light reflected directly from the Sun enters our eyes. When night falls and the interference from sunlight vanishes, you’ll see a beautiful light pillar twinkling more clearly than the Milky Way crossing the sky. Recognizing this outstanding creativity, the novel *Ringworld* won the Hugo Award in 1971. Although the author is widely considered great at setup but terrible at plot, leading to opinions that this was overrated, the concept of the Ringworld itself was undeniably fresh. However, MIT students even marched at the World SF Convention shouting, "The Ringworld is unstable!" The Ringworld is a structure rotating in place, not orbiting around a star. This means that if it collides with a celestial body or if solar winds slightly twist its orbit, it lacks the restoring force to return to its original path and would either collide with the star or collapse due to gravitational misalignment. This feedback was delivered to the author, and in subsequent works, the setting was updated to include attitude control thrusters that constantly correct the orbit in real-time. Besides this, questions arise about what material would be needed to maintain the strength of such a massive ring while rotating it at such high speeds. Larry Niven created a hypothetical material called Scrith, which, in other words, means no material currently known to mankind could build and maintain the Ringworld.
Anyway, the unique concept of the Ringworld influenced many sci-fi works. For example, the *Halo* series was strongly influenced by Ringworld with its narrative of exploring mysterious ring-shaped structures, even though they are much smaller Bishop Ring structures.
It also directly appears as a gigantic space structure in the sci-fi strategy game *Stellaris*, where players can repair abandoned Ringworlds or build them themselves once they reach the late stages of the game. The sunshades aren't implemented, but they exist in the lore. ++
Extra: Ring-ring-ring-ring-ringworld
"The logistics are impossible! Where do you get the raw material? And if you manage it, one tiny asteroid will smash the whole thing to pieces. (But if we can build a Ringworld, we can probably handle a few space rocks.)"
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